This invention relates to a process for preparing carboxylic acid halides and carboxylate salts.
Carboxylic acid halides, for example trifluoroacetyl chloride, trifluoroacetyl bromide or trifluoroacetyl iodide, are valuable intermediates in chemical synthesis, for example in the preparation of herbicides, surfactants and pharmaceuticals. For example, trifluoroacetyl chloride is a polymerization initiator for tetrafluoroethylene.
Carboxylic acid salts are also valuable intermediates in synthesis. Thus, halogen-substituted aromatic compounds can be converted to the corresponding trifluoromethylated aromatic compound using sodium trifluoroacetate with elimination of sodium chloride and decarboxylation.
One industrial process for preparing trifluoroacetyl chloride is described in U.S. Pat. No. 4,382,897, in which 1,1,1,-trifluoro-2,2,2,-trichloroethane is reacted with sulfur trioxide in the presence of mercury salts and, additionally, boron halide and/or halosulfonic acid. The resulting CF.sub.3 C(O) Cl can be converted, for example by means of alcohols, to esters of trifluoroacetic acid. Carboxylic acid bromides and carboxylic acid iodides can be produced from the corresponding carboxylic acid chlorides by using anhydrous hydrogen bromide or hydrogen iodide, see R. N. Haszeldine, J. Chem. Soc. 1951, pages 584 to 587. Another industrial process for preparing trifluoroacetyl bromide and trifluoroacetyl iodide is disclosed in Japanese Patent Application No. JP-A 2/262 530. In this process the corresponding acetyl fluoride is reacted with lithium bromide or lithium iodide. In addition, it is known to prepare trifluoroacetyl halides by reacting trifluoroacetic acid anhydride with phosphoric acid chlorides or phosphinic acid chlorides. A mixed anhydride of phosphonic acid or phosphinic acid and trifluoroacetic acid is formed as a by-product, which is industrially worthless. This process is described by J. Helinski et al. in Phosphorus Sulfur Silicon Relat. Chem. 54 (1990), pages 225 and 226. Thus, the known processes, especially for preparing carboxylic acid halides, are technically difficult to carry out, or they form unwanted waste products.